A typical variable displacement refrigerant compressor of the variable angle wobble plate type is disclosed, e.g., by the U.S. Pat. No. 4,428,718, wherein its displacement or capacity is varied automatically according to air conditioning demand by controlling the refrigerant gas pressure differential between the crankcase and suction chamber by means of a control valve which is actuated by a bellows operable in response to suction pressure of the refrigerant gas. According to this prior art, the bellows is so arranged that, when the suction pressure is dropped to a predetermined control point, it acts on the control valve in such a way that the latter is brought to a position where a communication passage between the crankcase and suction chamber is closed and, simultaneously, another passage for establishing communication between the discharge chamber and crankcase is opened to elevate the crankcase pressure. As a result, the above crankcase-suction pressure differential is increased, thus causing the compressor to operate at a reduced displacement while preventing the suction pressure from being dropped beyond a set level.
With the compressor wherein the displacement control valve is thus actuated by the bellows operable in response to the suction pressure, however, if a rapid drop takes place in suction pressure because of, e.g. accelerating operation, the control valve will be actuated by the bellows then responding to such drop of the suction pressure. Thus, the compressor is brought into operation at a reduced displacement only by a decrease of the suction pressure without increasing the crankcase pressure. As a result of the above actuation of the control valve, however, the passage between the discharge chamber and crankcase is opened to admit compressed high pressure gas into the crankcase, thereby crankcase pressure being built up to an excessive level. When the compressor speed is reduced to a normal level after completion of the above acceleration, however, the compressor tends to operate at a displacement which is insufficient for the cooling capacity demand then increased so as to compensate for the capacity insufficiently resulting from the acceleration during which the compressor displacement was reduced. It is because the suction pressure is increased with the decrease of the compressor speed and also with the above increased cooling capacity demand and, therefore, the crankcase-suction pressure differential prevailing after such acceleration is not large enough to restore the wobble plate rapidly to its full stroke position and the excessively elevated crankcase pressure can be reduced only at a slow rate. As a result, not only the temperature in the automobile's interior is elevated, but also it takes a long time before the optimum temperature is reached because it is necessary to move the wobble plate to its maximum angle position for decreasing the ambient temperature again to the optimum level. Furthermore, because an excessive high crankcase pressure results whenever the compressor speed is accelerated, there is a fear that sealing surfaces of shaft seals disposed in the crankcase may be deteriorated by frequent variation of the crankcase pressure.
There has been another disadvantage with the conventional compressor in that, once the wobble plate is brought to its zero displacement or non-compression position with the inceasing crankcase-suction pressure differential, the wobble plate is unable to release itself from such zero displacement state for restoration to a position of, e.g., about 20 degree or more. This means that the wobble plate requires any means for urging the same toward its full displacement position, which will not only make the compressor mechanism more complicated in construction, but also restrict the range controllable by the pressure differential.
Furthermore, the compressor which vents the discharge chamber to the crankcase is disadvantageous because the compression efficiency is decreased by part of the compressed high pressure refrigerant gas escaping from the discharge chamber into the crankcase and also a costly three-way valve is used as the displacement control valve.